Method of forming a heterojunction of a carbon nanotube and a different material, method of working a filament of a nanotube

ABSTRACT

A carbon nanotube is contacted with a reactive substance which is a metal or a semiconductor. The reactive substance is heated to diffuse atoms of the reactive substance into the carbon nanotube so that the carbon nanotube is partially transformed or converted into carbide as a reaction product. Thus, a heterojunction of the reaction product and the carbon nanotube is formed. For example, the carbon nanotube ( 2 ) is contacted with a silicon substrate ( 1 ). The silicon substrate ( 1 ) is heated to cause solid-solid diffusion of Si. As a result, SiC ( 3 ) is formed as the heterojunction. At least a part of a filament material of a carbon nanotube is irradiated with electromagnetic wave to deform the filament material.

[0001] This is a divisional of application Ser. No. 09/736,220 filedDec. 15, 2000, which is a divisional of prior application Ser. No.09/327,510 (U.S. Pat. No. 6,203,864) filed Jun. 8, 1999; the disclosureof each is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] This invention relates to a method of forming a heterojunction ofa carbon nanotube and a different material and, in particular, to amethod of forming a heterojunction of a carbon nanotube and carbide.

[0003] This invention relates also to a filament, a method of inducingan electric current therein, and a method of working the same and, inparticular, to a filament having a nanostructure and adapted for use ina micromachine and an electron source, a method of inducing an electriccurrent therein, and a method of working the same.

[0004] A so-called heterojunction formed by heterogeneous or differentmaterials is an important structure in order to utilizematerial-specific characteristics in an electronic device.

[0005] A carbon nanotube comprises a graphite sheet composed ofsix-member carbon rings and has a cylindrical structure formed byrolling the graphite sheet in a manner such that the six-member carbonrings are aligned in a helical fashion.

[0006] The carbon nanotube, together with a spherical fullerenerepresented by C₆₀, is expected as a useful material for an electronicdevice because of its specific electric characteristics. Particularly,attention is directed to a bond of the carbon nanotube and carbide.

[0007] This is because carbide itself has very interesting electriccharacteristics. For example, SiC has semiconducting features. TiC hasmetallic features. Fe₃C acts as a ferromagnetic material. NbC attractsthe attention as a superconducting material. BC_(x) serves as aninsulator. Thus, carbide has a wide variety of electric characteristics.

[0008] On the other hand, a single-wall carbon nanotube has specificelectric characteristics. That is, the single-wall carbon nanotube actsas a semiconductor or a metal in dependence upon a diameter and ahelical condition (an angle formed between an axial direction of thenanotube and an aligning direction of carbon atoms) (M. S. Dresselhauset al “Science of Fullerenes and Carbon Nanotubes” (Academic Press, NewYork, 1996)). It is expected that various functional devices can beachieved by a combination of carbide and the carbon nanotube.

[0009] However, no conventional technique exists to form suchheterojunction of carbide and the carbon nanotube. This is because thecarbon nanotube has a very high Young's modulus and is thereforedifficult to be mechanically processed or deformed.

[0010] In order to produce a carbide nanorod using the carbon nanotubeas a starting material, use has been made of a technique of contacting amultiwall carbon nanotube with volatile oxide such as SiO and B₂O₂ orhalide such as SiI₄, TiI₄, NbI₄, and FeCl₃ to cause high-temperaturereaction (H. Dai et al “Synthesis and characterization of carbidenanorods”, Nature, Vol. 375, pp. 769-772, (1995); D. Zhou et al“Production of silicon carbide whiskers from carbon nanoclusters”, Chem.Phys. Lett., Vol. 222, pp. 233-238 (1994); W. Han et al “Continuoussynthesis and characterization of silicon carbide nanorods”, Chem. Phys.Lett., Vol. 265, pp. 374-378 (1997)). Another technique is disclosed inEP 60388 A2 (1993) in which carbon fiber is transformed or convertedinto a SiC rod by the use of SiO vapor.

[0011] In the above-mentioned techniques of producing the carbidenanorod by the use of vapor-solid reaction, the carbon nanotube isexposed to reactive vapor to transform a whole of the carbon nanotubeinto carbide. Therefore, those techniques can not be applied toformation of the heterojunction. In other words, in order to realize theheterojunction, a part of the carbon nanotube must be selectivelytransformed into carbide with a remaining part protected from thereaction. However, no conventional technique can achieve such selectivereaction.

[0012] Since a single-wall carbon nanotube (SWCNT) having ananostructure has been discovered (Iijima et al, “Pentagons, heptagonsand negative curvature in graphite microtubule growth”, Nature, vol.356, p776, (1992)), physical properties of the SWCNT are graduallyrevealed and research and development for practical applications areactively carried out.

[0013] The SWCNT comprises a hexagonal network graphite plane rolledinto a cylindrical shape. The SWCNT has an electron structure widelyvaried depending upon a tube diameter and a chiral angle. Therefore, theelectric conductivity of the SWCNT is variable between that of a metaland that of a semiconductor. The SWCNT is believed to have a featuresimilar to one-dimensional electric conductivity.

[0014] For example, the SWCNT is applicable to a filament having ananostructure. For use as the filament, the SWCNT must be deformed intoa desired shape. A technique of selectively deforming the SWCNT isexpected to be useful in application to micromachines and infacilitating the preparation of high-resolution probes (see H. Dai et al“Nanotubes as nanoprobes in scanning probe microscopy”, Nature, Vol.384, pp. 147-150 (1996) and S. S. Wong et al “Covalently functionalizednanotubes as nanometre-sized probes in chemistry and biology”, Nature,Vol. 394, pp. 52-55 (1998)).

[0015] On the other hand, a technique of selectively feeding an electriccurrent to the filament having a nanostructure, such as the SWCNT, showsa possibility of development of electronic devices having amicrostructure (S. J. Tans et al “Room-temperature transistor based on asingle carbon nanotube”, Nature, Vol. 393, pp. 49-52 (1998)). Inaddition, this technique is useful as one of the high-resolutiontechniques in analysis evaluation. Thus, this field of technique ishighly expected.

[0016] To meet such expectation, proposal is made of a filament of afield emission type (Jean-Marc Bonard et al “Field emission fromsingle-wall carbon nanotube films”, Appl. Phys. Lett. Vol. 73, pp.918-920 (1998)). The filament comprises a plurality of SWCNT filamentsscattered over a plurality of electrodes formed on a substrate. Byapplying a predetermined voltage between the electrodes, electrons areemitted from the filaments.

[0017] As compared with a typical thermionic emission type, theabove-mentioned filament is advantageous in the following respects.Specifically, heating is unnecessary so that energy efficiency is high.The filament comprises carbon atoms alone and is manufactured at a lowcost. In recent years, much attention is directed to this field oftechnique.

[0018] In order to individually and selectively deform the filament, forexample, a manipulation technique is necessary. Manipulation of thosefilaments using the SWCNTs and having a nanostructure requires highresolution comparable to that required in manipulation of atoms.Therefore, it is in fact impossible to selectively deform the filament.

[0019] In addition, there is no existing technique of selectivelyinducing an electric current in the filament of a nanostructure. Thus,it is impossible to selectively induce the electric current in thefilament using the SWCNT having a nanostructure.

SUMMARY OF THE INVENTION

[0020] It is therefore an object of this invention to provide a methodof forming a heterojunction of a carbon nanotube and carbide, which isuseful for an electronic device.

[0021] It is therefore an object of this invention to provide a filamentsuch as a SWCNT having a nanostructure which can be individually andselectively deformed into a desired shape.

[0022] It is another object of this invention to provide a filament suchas a SWCNT having a nanostructure in which an electric current can beselectively induced.

[0023] It is still another object of this invention to provide a methodof inducing an electric current in the above-mentioned filament.

[0024] It is yet another object of this invention to provide a method ofselectively deforming the filament.

[0025] According to this invention, there is provided a method ofproducing a heterojunction of a carbon nanotube and carbide, wherein apart of the carbon nanotube is contacted with a reactive substance tocause reaction of the carbon nanotube and the reactive substance bysolid-solid diffusion.

[0026] With the above-mentioned method, the reaction of the carbonnanotube is restricted to a contacting area where the carbon nanotube iscontacted with the reactive substance and an adjacent zone around thecontacting area. In a most part of a noncontacting area, the carbonnanotube is not changed in structure. Therefore, it is possible to forma heterojunction of the carbon nanotube and carbide.

[0027] According to this invention, there is provided a filamentcomprising a filament material which is deformed by irradiation ofelectromagnetic wave to at least a part thereof.

[0028] Preferably, the filament material is a nanotube.

[0029] Preferably, the nanotube is a single-wall nanotube.

[0030] Preferably, the nanotube has a bundled structure.

[0031] Preferably, the nanotube is a carbon nanotube.

[0032] According to this invention, there is provided a method ofinducing an electric current in a filament, comprising the step ofirradiating at least a part of a filament material with electromagneticwave to selectively induce the electric current in the filamentmaterial.

[0033] According to this invention, there is provided a method ofworking a filament, comprising the step of irradiating at least a partof a filament material with electromagnetic wave to deform the filamentmaterial.

[0034] Preferably, the electromagnetic wave is visible light.

BRIEF DESCRIPTION OF THE DRAWING

[0035]FIGS. 1A and 1B are schematic views for describing a method offorming a heterojunction according to this invention;

[0036]FIG. 2A is an electron micrograph showing a heterojunction of abundle of a plurality of single-wall carbon nanotubes and SiC; and

[0037]FIG. 2B is an electron micrograph showing a heterojunction of asingle-wall carbon nanotube and SiC.

[0038]FIGS. 3A and 3B are optical micrographs for describing a method ofworking a filament according to one embodiment of this invention;

[0039]FIGS. 4A and 4B are optical micrographs showing deformation of thefilament illustrated in FIGS. 3A and 3B at different levels ofirradiation energy;

[0040]FIG. 5 is an optical micrograph showing the filament illustratedin FIGS. 3A and 3B irradiated with a laser beam; and

[0041]FIG. 6 is a view showing the result of measurement of an electriccurrent induced in the filament illustrated in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] Now, description will be made about a preferred embodiment ofthis invention with reference to the drawing.

[0043] As a reactive substance, use is made of a metal such as Ti, W,Mo, V, Fe, and Nb or a semiconductor such as Si. A part of a carbonnanotube is contacted with the reactive substance. Preferably, thereactive substance is held in a vacuum or an inactive gas. At least thereactive substance is heated to diffuse the reactive substance towardsthe carbon nanotube. Thus, the reaction between the carbon nanotube andthe reactive substance proceeds.

Embodiment

[0044] A heterojunction of a single-wall carbon nanotube and SiC wasformed by the use of single crystal silicon as a reactive substance. Atfirst, a (111) plane silicon wafer was cut into a dimension of about 7mm long and about 3 mm wide in a direction perpendicular to a substrate.Thereafter, mechanical polishing was performed until the thickness of acenter portion is reduced to the order of several tens of microns. Then,chemical etching was performed until the thickness is further reduced tothe order of several tens of nanometers. Thus, a silicon substrate 1 wasprepared. As an etchant, a mixed solution of HF and HNO3 (HF: HNO3=1:4)was used. An oxide film on the surface of the silicon substrate wasremoved by chemical etching.

[0045] A number of single-wall carbon nanotubes 2 prepared by laserablation were dispersed in ethanol and put on the silicon substrate 1prepared as described above. In this event, most of the carbon nanotubes2 were extended, heavily bent, or bundled together. After ethanol wasevaporated from the silicon substrate 1, each of the carbon nanotube 2was placed on the silicon substrate 1 with its three-dimensionalstructure maintained. As a result, the silicon substrate 1 and thecarbon nanotube 2 were partially contacted in a small area (FIG. 1A).Thus, a sample was prepared.

[0046] Then, the sample was mounted on a heating stage of aultra-high-vacuum transmission electron microscope (UHV-TEM,JEM-2000FXVII). A vacuum chamber was evacuated to a pressure between10-9 and 10-8 Torr.

[0047] The silicon substrate 1 was directly supplied with an electriccurrent to heat the sample. As a result of measurement by a pyrometer,the highest temperature of the silicon substrate 1 was about 1000° C.When the temperature became higher than about 800° C., surface migrationof silicon was observed.

[0048] By observation through the transmission electron microscope(TEM), it was confirmed that heating for several minutes caused localreaction of silicon and the single-wall carbon nanotube to produce SiC 3(FIG. 1B). Heating was carried out for different heating periodscontrollably varied within a range from several minutes to one hour.However, no difference in appearance was observed in dependence upon theheating periods. The heterojunction of the single-wall carbon nanotubeand SiC thus obtained was shown in each of FIGS. 2A and 2B as amicrograph taken by the TEM. FIG. 2A shows the heterojunction of abundle of a plurality of single-wall carbon nanotubes and SiC while FIG.2B shows the heterojunction of one single-wall carbon nanotube and SiC.

[0049] Although the preferred embodiment has been described in theforegoing, this invention is not restricted thereto but can be modifiedin various manners within the scope of this invention. For example, notonly the single-wall carbon nanotube but also a multiwall carbonnanotube can be used. In the foregoing embodiment, heating was performedby feeding the electric current to the substrate. Alternatively, theelectric current may be supplied between the carbon nanotube and thesubstrate. Instead of the electric current, use may be made of any otherheating means such as infrared radiation. The heating may be performednot only in the vacuum but also in an argon or a nitrogen atmosphere.

[0050] As described above, the method according to this inventioncomprises the step of partially contacting the carbon nanotube with thereactive substance to cause the reaction between carbon nanotube and thereactive substance by solid-solid diffusion of the reactive substance.Thus, by the above-mentioned method which is very simple, it is possibleto selectively form the heterojunction between a part of the carbonnanotube and carbide. The heterojunction of the carbon nanotube andcarbide achieved by this invention is useful in formation of electronicdevices and will make a great contribution to electronic industry.

[0051] Now, description will be made about a filament according to oneembodiment of this invention as well as a method of inducing an electriccurrent in the filament and a method of working the filament.

[0052] The filament having a nanostructure comprises a filament materialon the order of nanometers. At least a part of the filament material isirradiated with electromagnetic wave such as visible light to deform thefilament material into a desired shape, for example, an arcuate shape ora η shape.

[0053] As the filament material, use is advantageously made of ananotube (NT), particularly, a single-wall carbon nanotube (SWCNT) or aplurality of SWCNTs in a bundled structure.

[0054] Next, the method of working the filament will be described.

[0055] At first, a SWCNT as the filament material is formed by laserablation known in the art (see Y. Zhang et al “Microscopic structure ofas-grown single-wall carbon nanotubes by laser ablation”, PhilosophicalMagazine Letters, Vol. 78, No. 2, pp. 139-144 (1998)).

[0056] Specifically, a graphite target containing 1.2at % of Ni and Coas catalysts was placed in a furnace heated to 1200° C., kept at apressure of 500 Torr, and supplied with an Ar gas at a flow rate ofabout 300 sccm. By the use of a Nd-doped YAG (yttrium-aluminum-garnet)laser, the graphite target was irradiated with second harmonic producedby the YAG laser to obtain the SWCNT. The second-order harmonic wave hasa pulse width of about 8 ns and energy per pulse of about 3 J/cm².

[0057] Then, the SWCNT was put in a sample cell having a quartz windowwith two electrodes arranged inside.

[0058] Within the sample cell, the SWCNT guided by a stream of the Argas was caught between the electrodes. Thereafter, the sample cell isevacuated to a pressure of 0.1 Torr.

[0059] Next, the SWCNT was irradiated through the quartz window of thesample cell with visible light emitted from a halogen lamp of 150 W toobtain a filament deformed into a desired shape.

[0060] Referring to FIGS. 3A and 3B, the filament before and afterirradiation of the visible light is shown, respectively. Herein, thevisible light had irradiation energy of about 20 mW/cm².

[0061] As will be understood from FIGS. 3A and 3B, the filament can bedeformed into a desired shape by simply irradiating the filament withthe visible light. The deformation of the filament can be controlled byselecting the irradiation energy of the visible light and theirradiation area of the filament.

[0062] Referring to FIGS. 4A and 4B, the deformation of the filament isdependent upon the level of the irradiation energy. In FIG. 4A, thevisible light had the irradiation energy of 30 mW/cm². In FIG. 4B, thevisible light had the irradiation energy of 5 mW/cm².

[0063] As seen from FIGS. 4A and 4B, the deformation of the filamentapparently depends upon the level of the irradiation energy.

[0064] Referring to FIG. 5, the filament was irradiated with a laserbeam having a wavelength of 632 nm and irradiation energy of about 800mW/cm² by the use of a He—Ne laser. The deformation of the filament wassubstantially equivalent to that of the filament illustrated in FIG. 4A.

[0065] Referring to FIG. 6, an electric current was induced in thefilament when the filament was irradiated at the center between twoelectrodes with a laser beam having a wavelength of 632 nm andirradiation energy of about 800 mW/cm² by the use of a He—Ne laser. Theresult of measurement of the electric current is illustrated in thefigure.

[0066] As seen from FIG. 6, it is possible to selectively induce theelectric current in the filament by irradiating the filament with thelaser beam.

[0067] According to this embodiment, it is possible to individually andselectively deform the filament of a nanostructure such as SWCNT byirradiating the filament with electromagnetic wave such as visiblelight.

[0068] It is also possible to induce the electric current in thefilament by irradiating the filament with electromagnetic wave such asvisible light.

[0069] In each of the above-mentioned operations, it is sufficient tosimply irradiate at least a part of the filament with theelectromagnetic wave such as visible light. Thus, the operation is verysimple and convenient. The filament can be applied as a nanoelectronicselement.

[0070] In the foregoing, description has been made about one embodimentof the filament, the method of inducing the electric current in thefilament, and the method of working the filament. However, thisinvention is not restricted to the foregoing embodiment but can bemodified in various manners within the scope of this invention.

[0071] For example, the halogen lamp and the He—Ne laser are used in theforegoing embodiment. However, similar effect is achieved by the use ofany other light or energy source.

[0072] As described above, according to this invention, it is possibleto individually and selectively deform the filament of a nanostructuresuch as the SWCNT into a desired shape.

[0073] By irradiating the filament with electromagnetic wave such asvisible light, it is possible to selectively induce the electric currentin the filament.

What is claimed is:
 1. A method of inducing an electric current in afilament material, comprising the step of irradiating at least a part ofsaid filament material with electromagnetic wave to selectively inducethe electric current in said filament material.
 2. A method of working afilament material, comprising the step of irradiating at least a part ofsaid filament material with electromagnetic wave to deform said filamentmaterial.
 3. A method as claimed in claim 2, wherein saidelectromagnetic wave is visible light.
 4. A method as claimed in claim1, wherein said filament material is of a carbon nanotube.
 5. A methodas claimed in claim 2, wherein said filament material is of a carbonnanotube.